Systems and methods for reducing the influence of plasma-generated debris on the internal components of an EUV light source

ABSTRACT

Systems and methods are disclosed for reducing the influence of plasma generated debris on internal components of an EUV light source. In one aspect, an EUV metrology monitor is provided which may have a heater to heat an internal multi-layer filtering mirror to a temperature sufficient to remove deposited debris from the mirror. In another aspect, a device is disclosed for removing plasma generated debris from an EUV light source collector mirror having a different debris deposition rate at different zones on the collector mirror. In a particular aspect, an EUV collector mirror system may comprise a source of hydrogen to combine with Li debris to create LiH on a collector surface; and a sputtering system to sputter LiH from the collector surface. In another aspect, an apparatus for etching debris from a surface of a EUV light source collector mirror with a controlled plasma etch rate is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/174,442, filed Jun. 29, 2005, which is a continuation-in-partapplication of U.S. patent application Ser. No. 10/979,945, entitled LPPEUV LIGHT SOURCE, filed on Nov. 1, 2004, Attorney Docket No.2004-0088-01, and is a continuation-in-part of application of U.S.patent application Ser. No. 10/900,839, entitled EUV LIGHT SOURCE, filedon Jul. 27, 2004, Attorney Docket No. 2004-0044-01, and is acontinuation-in-part of application of U.S. patent application Ser. No.10/803,526, entitled HIGH REPETITION RATE LPP EUV LIGHT SOURCE, filed onMar. 17, 2004, Attorney Docket No. 2003-0125-01, and is acontinuation-in-part application of U.S. patent application Ser. No.10/798,740, entitled COLLECTOR FOR EUV LIGHT, filed on Mar. 10, 2004,Attorney Docket No. 2003-0083-01, the disclosures of each of which arehereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to extreme ultraviolet (“EUV”) lightgenerators providing EUV light from a plasma created from a sourcematerial and collected and directed to a focus for utilization outsideof the EUV light source generation chamber, e.g., for semiconductorintegrated circuit manufacturing photolithography e.g., at wavelengthsof around 50 nm and below.

BACKGROUND OF THE INVENTION

Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiationhaving wavelengths of around 50 nm or less (also sometimes referred to asoft x-rays), and including light at a wavelength of about 13.5 nm, canbe used in photolithography processes to produce extremely smallfeatures in substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has an element, e.g.,xenon, lithium or tin, with an emission line in the EUV range. In onesuch method, often termed electric discharge produced plasma (“DPP”),the plasma may be produced by an electrical discharge between a pair ofelectrodes. In another method, the required plasma can be produced byirradiating a target material, such as a droplet, stream or cluster ofmaterial having the required line-emitting element, with a laser beam.This later process is referred to as laser produced plasma (“LPP”).

For each of these processes, the plasma is typically produced in asealed vessel, e.g., vacuum chamber, and monitored using various typesof metrology equipment. In addition to generating EUV radiation, theseplasma processes also typically generate undesirable by-products in theplasma chamber which can include heat, high energy ions and scattereddebris from the plasma formation, e.g., atoms and/or clumps of sourcematerial that is not fully ionized in the plasma formation process.

These plasma formation by-products can potentially damage or reduce theoperational efficiency of the various plasma chamber optical elementsincluding, but not limited to, collector mirrors including multi-layermirrors (MLM's) capable of EUV reflection at normal incidence andgrazing angle incident mirrors, the surfaces of metrology detectors,windows used to image the plasma formation process, and in the case ofLPP, the laser input window. The heat, high energy ions and/or sourcematerial debris may be damaging to the optical elements in a number ofways, including heating them, coating them with materials which reducelight transmission, penetrating into them and, e.g., damaging structuralintegrity and/or optical properties, e.g., the ability of a mirror toreflect light at such short wavelengths, corroding or eroding themand/or diffusing into them. In addition, some optical elements, e.g.,the laser input window, form a part of the vacuum chamber and are thusplaced under a stress when a vacuum is present in the plasma chamber.For these elements, deposits and heat can combine to fracture (i.e.,crack) the element resulting in a loss of vacuum and requiring a costlyrepair.

Accessing contaminated or damaged optical elements in the plasma chamberfor the purpose of cleaning or replacing the elements can be expensive,labor intensive and time-consuming. In particular, these systemstypically require a rather complicated and time consuming purging andvacuum pump-down of the plasma chamber prior to a re-start after theplasma chamber has been opened. This lengthy process can adverselyaffect production schedules and decrease the overall efficiency of lightsources for which it is typically desirable to operate with little or nodowntime.

With the above in mind, Applicants disclose systems and methods forreducing the influence of plasma-generated debris on the internalcomponents of an EUV light source.

SUMMARY OF THE INVENTION

An EUV metrology monitor for an EUV light source which generates debrisby plasma formation is disclosed. The monitor may comprise a radiationdetector; an element for filtering radiation and directing filteredradiation to the detector, the element positioned at a location whereindebris generated by plasma formation is deposited on the element; and aheater to heat the element to a temperature sufficient to remove atleast a portion the deposited debris.

In another aspect of an embodiment of the present invention, a device isdisclosed for removing plasma generated debris from an EUV light sourcecollector mirror. For the device, the collector mirror may be positionedrelative to a plasma formation site to cause a different debrisdeposition rate at different zones on the collector mirror. The devicemay comprise a first heating system for heating a first zone of thecollector mirror to a first temperature, T₁, to remove debris therefrom;and a second heating system for heating a second zone of the collectormirror to a second temperature, T₂, to remove debris therefrom, withT₁≠T₂.

In yet another aspect of an embodiment of the present invention, asystem is disclosed for protecting an EUV light source detector surfacefrom plasma generated debris. The system may comprise at least onehollow tube having a tube wall that surrounds a tube lumen, the tubebeing interposed between a plasma formation site and the detectorsurface and oriented to prevent at least a portion of the debrisdirected toward the detector surface from reaching the surface andallowing at least a portion of light generated at the plasma formationsite to pass through the lumen and reach the detector surface; and aheater for heating the tube wall to remove debris deposited thereon.

In one aspect of an embodiment of the present invention, a collectormirror system for use with an EUV light source that generates Li debrisby plasma formation is. disclosed. The collector mirror system maycomprise a source of hydrogen to combine with Li debris to create LiH ona surface of the collector; and a sputtering system for directingsputtering molecules toward the collector surface to sputter LiH fromthe collector surface.

In still another aspect of an embodiment of the present invention, anapparatus for etching debris from a surface of an EUV light sourcecollector mirror with a controlled plasma etch rate is disclosed. Thesystem may comprise a plasma etch system for etching debris with theetch system having at least one controllable parameter to vary a plasmaetch rate; a reference material having a surface positioned to receivesubstantially the same amount of debris accumulation as at least onezone on the collector mirror surface; an instrument for analyzingetching plasma emission from the reference material surface to producean output indicative of a debris accumulation amount on the referencematerial surface; and a controller responsive to the output to vary anetch rate parameter to control plasma etch rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for alaser-produced plasma EUV light source according to an aspect of thepresent invention;

FIG. 2 shows a schematic, side view of an aspect of an embodiment of ashield system for protecting a plasma chamber optical element fromplasma source material debris;

FIG. 3 shows a schematic, side view of a plurality of hollow tubesillustrating the path of an exemplary light ray through a hollow tubeand the path of an exemplary debris particle being captured by a hollowtube;

FIG. 4 shows a schematic, sectional view of an aspect of an embodimentof the present invention wherein an EUV metrology monitor may comprise aheater to heat a filter foil to remove deposited plasma generateddebris;

FIG. 5 shows a schematic, sectional view of another aspect of anembodiment of the present invention wherein an EUV metrology monitor maycomprise a heater to heat a multi-layer mirror to remove depositedplasma generated debris;

FIG. 6 illustrates an aspect of an embodiment of the present inventionin which different zones of a collector mirror are etched to removeplasma generated debris at different etch rates;

FIG. 7 illustrates another aspect of an embodiment of the presentinvention in which different zones of a collector mirror may be heatedat different rates to remove plasma generated debris at differentremoval rates; and

FIG. 8 illustrates another aspect of an embodiment of the presentinvention in which an apparatus for etching debris from a surface of aEUV light source collector mirror with a controlled plasma etch rate maybe provided.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to FIG. 1 there is shown a schematic view of an exemplaryproduction EUV light source, e.g., a laser produced plasma EUV lightsource 20 according to an aspect of the present invention. Althoughaspects of the present invention are illustrated with reference to alaser produced plasma (LPP), it is to be appreciated that the presentinvention is equally applicable to other types of light sources whichproduce a plasma including an electric discharge produced plasma(“DPP”), a representative construction of which is disclosed in co-ownedU.S. Pat. No. 6,815,700, which is hereby incorporated by reference.

Continuing with FIG. 1, an LPP light source 20 may contain a pulsedlaser system 22, e.g., a gas discharge excimer or molecular fluorinelaser operating at high power and high pulse repetition rate and may bea MOPA configured laser system, e.g., as shown in U.S. Pat. Nos.6,625,191, 6,549,551, and 6,567,450. The light source 20 may alsoinclude a target delivery system 24, e.g., delivering targets in theform of liquid droplets, a liquid stream, solid particles or clusters,solid particles contained within liquid droplets or solid particlescontained within a liquid stream. The targets may be delivered by thetarget delivery system 24, e.g., into the interior of a chamber 26 to aplasma formation site 28.

Laser pulses may be delivered from the pulsed laser system 22 along alaser optical axis through a laser input window 57 and into the chamber26 to the irradiation site, suitably focused, to create a plasma, havingcertain characteristics which depend on the source material of thetarget. These characteristics may include the wavelength of the EUVlight produced and the type and amount of debris released from theplasma.

The light source may also include a collector 30, e.g., a reflector,e.g., in the form of a truncated ellipse, with an aperture to allow thelaser light to pass through and reach the ignition site 28. Thecollector 30 may be, e.g., an elliptical mirror that has a first focusat the ignition site 28 and a second focus at a so-called intermediatepoint 40 (also called the intermediate focus 40) where the EUV light isoutput from the light source and input to, e.g., an integrated circuitlithography tool (not shown).

The pulsed system 22 may include a dual chamber, e.g., a masteroscillator-power amplifier (“MOPA”), gas discharge laser system having,e.g., an oscillator laser system 44 and an amplifier laser system 48,with, e.g., a magnetic reactor-switched pulse compression and timingcircuit 50 for the oscillator laser system 44 and a magneticreactor-switched pulse compression and timing circuit 52 for theamplifier laser system 48, along with a pulse power timing monitoringsystem 54 for the oscillator laser system 44 and a pulse power timingmonitoring system 56 for the amplifier laser system 48. The system 20may also include an EUV light source controller system 60, which mayalso include, e.g., a target position detection feedback system 62 and afiring control system 65, along with, e.g., a laser beam positioningsystem 66.

The system 20 may also include a target position detection system whichmay include one or more droplet imagers 70 that provide an outputindicative of the position of a target droplet, e.g., relative to theignition site and provide this output to the target position detectionfeedback system, which can, e.g., compute a target position andtrajectory, from which a target error can be computed, if not on adroplet by droplet basis then on average. The target error may then beprovided as an input to the system controller 60, which can, e.g.,provide a laser position, direction and timing correction signal, e.g.,to the laser beam positioning system 66 that the laser beam positioningsystem can use, e.g., to control the laser timing circuit and/or tocontrol the laser position and direction changer 68, e.g., to change thefocus point of the laser beam to a different ignition point 28.

The target delivery control system 90, in response to a signal from thesystem controller 60 may, e.g., modify the release point of the targetdroplets as released by the target delivery mechanism 92 to correct forerrors in the target droplets arriving at the desired ignition site 28.An EUV light source detector 100 may also provide feedback to the systemcontroller 60 that can be, e.g., indicative of the errors in such thingsas the timing and focus of the laser pulses to properly intercept thetarget droplets in the right place and time for effective and efficientEUV light production.

As shown schematically in FIG. 1 and described in more detail below, anaspect of an embodiment of the present invention can include a shieldingsystem 102 for protecting a surface of a plasma chamber optical elementfrom debris generated at the plasma formation site 28. Although theshielding system 102 is shown positioned to protect a surface of an EUVlight source detector 100, it is to be appreciated that the shieldingsystem 102 can be used to protect other optical elements in the chamber26.

FIG. 2 shows in more detail a system 102, for protecting a surface 104of an optical element, e.g., EUV light detector 100, from plasmagenerated debris. As shown, the system 102 may include a plurality ofhollow tubes 126, e.g., so-called capillary tubes, with each tube havinga tube wall that surrounds a tube lumen (i.e., bore). Tubes 126 may bemade of a material, e.g., glass, metal or ceramic, e.g., borosilicatematerial, which reflects EUV light at grazing angles of incidence, e.g.,grazing incidence reflection at small (<10 degrees) angles of grazingincidence where the EUV reflectivity of smooth surfaces is relativelyhigh for most materials. As shown, the tubes 126 may be grouped togetherand housed within a stainless steel housing tube 128 having a similarshape as the tubes 126. In an exemplary embodiment, about 50 bent glasscapillary tubes 126 (1 mm outer diameter, 0.78 mm inner diameter, 150 mmlong) may be mounted inside of a bent stainless steel tube 128. As shownin FIG. 3, the tubes 126 may be shaped having a midsection 130 that maybe laterally offset from a tube axis 132 defined by the tube ends 134,136. In particular, the midsection 130 may be offset by a distance 138that is larger than inner diameter of the tube 126.

FIG. 3 shows that the tubes 126 may be interposed between the plasmaformation site 28 and the detector surface 104. FIG. 3 also shows anexemplary path 140 of an EUV light ray and the exemplary path 142 of adebris particle. As shown, the EUV light ray passes through the lumen(i.e., bore) of a tube 126 after one or more small angle grazingincidence reflections from the inner wall surface of the tube 126 andreaches the surface 104. On the other hand, as shown, the debrisparticle may strike the inner wall of the hollow tube and stick to theinner wall. Moreover, in some cases, the accumulation of debris on theinner wall may result in a surface that may be smooth enough toadequately reflect EUV light at grazing angles of incidence. Use of thetubes 126 may have an advantage over the use of flat mirrors to directlight to a detector in that they will direct the light towards the endof the tube and no complicated alignment is required, like in the caseof redirecting mirrors.

In use, the tubes 126 may be positioned inside the plasma chamber 26(see FIG. 1) and located between the plasma formation site 28 and anoptical element, e.g., detector 100, to thereby allow debris totemporarily deposit on the inner wall surfaces of the tubes 126. Asshown, detector 100 may include one or more thin EUV filter foils 146, amulti-layer mirror 148 and a photodiode detector 150.

Continuing with FIG. 2, the system 102 may include a heater 154 to heata portion of each tube 126, or in some cases each tube may be heated inits entirety, to a temperature sufficient to remove at least a portionthe deposited debris, e.g., to remove portions (or all) of one or moredeposited species. The application of heat may also function to smoothout deposits and thereby increase grazing angle reflections. Forexample, the heater may heat the tubes 126 to a temperature sufficientto vaporize at least a portion of a deposited material. For a plasmasource material which comprises Li, the heater 154 may be designed toheat the shield 108′ to a temperature in the range of about 400 to 550°C. to vaporize Li from the tube surface.

In some cases, the heater may heat the tubes 126 to a temperaturesufficient to initiate a chemical reaction between a deposited materialand an etchant gas that is introduced into the tubes 126. FIG. 2 showsthat the system 102 may include a sub-system 144 for releasing anetchant for flow into each tube 126. As shown, the sub-system 144 may bepositioned to release etchant for travel through the tubes 126 from thedetector 100 and toward the chamber 26. Suitable etchants can include,but are not necessarily limited to etchants such as HBr, Br₂, Cl₂, HCl,H₂, HCF₃ and combinations thereof. For example, an HBr concentration ofa few Torr can be used.

For a plasma source material which comprises Sn, the heater 154 may bedesigned to heat the tubes 126 (or portions thereof) to a temperature inthe range of about 200 to 325° C. to initiate a reaction between Sndeposits and one or more gaseous etchants, e.g., HBr, to create areaction product that may be removed from the inner tube wall.

In more structural detail, as shown in FIG. 2, the heater 154 maycomprise a heating element 156 that is wrapped around the tubes 126, anda current source 158 for passing a current through the heating element156. The heating element 156 may be made of a conductive material, andthus be heated via ohmic heating during current flow. Other means ofheating the tubes 126 may include, but are not limited to radiativeheaters, microwave heaters, RF heaters and combinations thereof.

FIG. 4 shows another aspect of an embodiment of the present inventionwhich may comprise an EUV metrology monitor 100′ having a detector 150′for measuring EUV light parameters, e.g., pulse energy or flux. In someapplications, it may be desirable for the detector to measure lighthaving a wavelength of about 13.5 nm and a bandwidth of about 2% orless. For this purpose, light from the EUV light source may be filteredat the monitor 100′. Specifically, as shown, the monitor 100′ maycomprise one or more filter foils 146 a′, 146 b′, 146 c′ and 146 d′, oneor more CaF₂ windows 160 a,b, and one or more multi-layer mirrors 148′capable of reflecting a band of light centered on 13.5 nm at normalincidence. It is to be appreciated that the multi-layer mirrors 148′,e.g., multilayer mirrors having alternating layers of MoSi₂ and Si, mayabsorb light, e.g., light outside the 2% band centered on 13.5 nm, andthus, may act as a band-pass optical filter. On the other hand, when aCaF₂ window 160 a,b is interposed along the beam path, EUV light may beabsorbed while UV and visible light may be transmitted through thewindow 160 a,b. Thus, the CaF₂ window 160 a,b may also act as an opticalfilter. Similarly, the filter foils 146 a′-d′, which may be comprised ofa thin layer of antimony, may absorb or reflect visible light whiletransmitting EUV radiation.

FIG. 2 further shows that the monitor 100′ may include a pair of linearmotion actuators 162 a,b to selectively interpose one or more filters146 a′-d′, 160 a,b along the beam path 164. The monitor 100′ may alsoinclude an entrance aperture 166 and fast shutter 168. With thisarrangement, the filters 146 a′-d′, 160 a,b may be undesirable exposedto plasma generate debris entering the monitor 100′ through the entranceaperture 166. In some cases, debris deposits may reduce the operationalefficiency of the filters 146 a′-d′, 160 a,b. With this in mind, themonitor 100′ may include a heater 170, which for the monitor 100′ thatis shown can be a radiative heater, to heat a filter 146 a′-d′, 160 a,bto remove plasma generated debris that has temporarily depositedthereon. Other means of heating the filters 146 a′-d′, 160 a,b mayinclude, but are not limited to ohmic heaters, radiative heaters,microwave heaters, RF heaters and combinations thereof.

For a plasma source material which comprises Li, the heater 170 may bedesigned to heat the filter(s) 146 a′-d′, 160 a,b to a temperature inthe range of about 400 to 550° C. to vaporize Li from the filtersurface. For a plasma source material which comprises Sn, the heater 170may be designed to heat the filter(s) 146 a′-d′, 160 a,b to atemperature in the range of about 200 to 325° C. to initiate a reactionbetween Sn deposits and gaseous etchants, e.g., HBr, to create areaction product that may be removed from the filter surface. Gaseousetchants can be introduced directly into the monitor 100′ or into thechamber 26 (See FIG. 1).

FIG. 5 shows an alternative arrangement for a monitor (generallydesignated monitor 100″). As shown, the EUV metrology monitor 100″ mayhave a detector 150″ for measuring EUV light parameters, e.g., pulseenergy or flux and may include one or more filters 146 a″, 146 b″, 146c″ and 146 d″, 160 a′,b′, one or more of which can be selectivelyinterposed along beam path 164′. The monitor 100″ may also include oneor more multi-layer mirrors 148″. It can be further seen that themonitor 100″ may also include an aperture 166′ and fast shutter 168′.With this arrangement, the multi-layer mirror 148″ may be undesirableexposed to plasma generate debris entering the monitor 100″ through theaperture 166′. Debris deposits may, in some cases, reduce theoperational efficiency of the mirror 148″. With this in mind, themonitor 100″ may include a heater 170′, which for the monitor 100′ thatis shown can be an ohmic heater that is mounted on the backside of themirror 148″, to heat the mirror 148″ and remove plasma generated debristhat has temporarily deposited thereon. Other means of heating themirror 148″ may include, but are not limited to radiative heaters,microwave heaters, RF heaters and combinations thereof.

For a plasma source material which comprises Li, the heater 170′ may bedesigned to heat the mirror 148″to a temperature in the range of about400 to 550° C. to vaporize Li from the mirror surface. For a plasmasource material which comprises Sn, the heater 170 may be designed toheat the mirror 148″ to a temperature in the range of about 200 to 325°C. to initiate a reaction between Sn deposits and gaseous etchants,e.g., HBr, to create a reaction product that may be removed from themirror surface. Gaseous etchants can be introduced directly into themonitor 100′ or into the chamber 26 (See FIG. 1).

In one aspect of an embodiment of the present invention, as illustratedby FIG. 1, a target material containing Lithium may be used to generatea plasma at the plasma formation site 28. With this arrangement, debriscontaining Lithium and Lithium compounds may deposit on the collectormirror 30. Inherently, Lithium is very reactive material and reacts withalmost any contaminant on a collector surface, and thus, creates lithiumcompounds. Typically, uncombined Lithium can be evaporated by heatingthe collector mirror 30 to an elevated temperature, e.g., 350-450° C. Inparticular, the temperature may be chosen to ensure that the Lithiumevaporation rate is higher than the rate of lithium debris deposition.Unfortunately, some Lithium compounds do not evaporate at these moderatetemperatures (i.e., 350-450° C.). For example, compounds such as Li₂O orLi₂CO₃ required higher temperatures to evaporate and do not easilysputter from the surface of the collector 30. To evaporate lithiumcompounds may require the collector to be heated to very hightemperature (above 600-700° C.) which may reduce or destroy thereflectivity of a typical multi-layer mirror. Thus, evaporation and orsputtering of lithium compounds may be problematic.

With the above in mind, FIG. 1 illustrates that a hydrogen source 200,e.g., a source of molecular or atomic hydrogen, e.g., atomic hydrogenfrom a remote plasma source, may be provided to introduce hydrogen intothe chamber 26 for reaction with Lithium to create LiH. A sputteringsystem 202 may be provided to generate sputtering ions and/or moleculesand direct them to the surface of the collector with sufficient energyto sputter LiH. For example, the sputtering system may establish an RFcleaning plasma, e.g., capacitive or inductively coupled, with helium orargon as the sputtering material. As shown, the collector 30 may be RFbiased to selectively control the energy of ions bombarding debris thathas deposited on the collector 30. It general, it can be significantlyeasier to sputter LiH from the collector surface than Li₂O or Li₂CO₃.Also LiH deposits may be more transparent than Li₂O. Sputtering in thismanner may be used alone to sputter Lithium and Lithium compounds or incombination with heat to evaporate Lithium and/or plasma etching.

FIG. 6 illustrates an aspect of an embodiment of the present inventionin which a laser 300 is focused to a plasma formation site 28′ in achamber 26′. A collector 30′, e.g., an elliptical collector having afirst focal point at or near the plasma formation site and a secondfocal point at an intermediary focus (See FIG. 1) may be provided. Withthis arrangement, plasma generated debris may deposit at different ratesat different zones on the collector mirror 30′. For example, more debrismay deposit at location 302 a than location 302 b (note, for anelliptical collector, location 302 b is farther from the plasmainitiation site 28′ than location 302 a). Thus, for the system shown inFIG. 6 which uses plasma etching to remove debris from the collector30′, a higher etch rate may be desirable at location 302 a than location302 b. (Note: it may be damaging to the mirror to continue etching aportion of the mirror after deposited debris has been removed). To thisend, the system may include a source 144′ of plasma etchant and firstand second, independently controllable, RF power supplies 304 a,b thatare attached respectively through capacitors to separate RF electrodes306 a,b, as shown. Although two RF systems are shown for respectivelyoperating on substantially annularly shaped collector zones, it is to beappreciated that more than two RF systems may be employed and the use ofRF systems is not limited to zones having any specific shape, such asthe annular shape shown.

Suitable etchants may include, but are not necessarily limited toetchants such as HBr, Br₂, Cl₂, HCl, H₂, HCF₃ and combinations thereof.A non-etching gas, e.g., Argon or Helium, may be introduced to establishthe etching plasma. As used herein, the term “plasma etching” means aprocess which may include one or more of the following process steps: 1)generation of reactive species in a plasma; 2) diffusion of thesespecies to the surface of the material being etched; 3) adsorption ofthese species on the surface; 4) occurrence of one or more chemicalreactions between the species and the material being etched, formingvolatile byproducts; 5) desorption of the byproducts from the surface;and 6) diffusion of the desorbed byproducts into the bulk of the gas.The embodiment shown in FIG. 6 can be used for target materialcontaining Lithium, tin, Xenon and/or other materials.

FIG. 7 illustrates another aspect of an embodiment of the presentinvention in which different zones of a collector 30″ may be heated atdifferent rates. Specifically, an etch rate may be strongly dependent ontemperature. For example, the rate of Tin removal using HBr and/or Br₂has been found to be strongly dependent on temperature in the range of150-400° C. As shown in FIG. 7, which shows the backside of an exemplaryelliptical collector 30″, differential heating may be employed usingohmic heating systems to establish different etch rates for differentcollector zones. Specifically, each heating system includes anelectrical power source 400 a,b connected to a respective, shapedconductor 402 a,b. Other types of heaters for heating collector zones todiffering temperatures may include, but are not limited to radiativeheaters, microwave heaters, RF heaters and combinations thereof. Theembodiment shown in FIG. 7 can be used for target material containingLithium, tin, Xenon and/or other materials.

FIG. 8 illustrates another aspect of an embodiment of the presentinvention in which an apparatus for etching debris from a surface of aEUV light source collector mirror 30″′ with a controlled plasma etchrate may be provided. As shown, the apparatus may include a referencematerial, e.g., witness plate 700, having a surface positioned toreceive a substantially same amount of debris accumulation as location702 on the surface of collector 30″′. For example, a small (about 1×1cm) sacrificial witness plate 700 may placed next to the MLM collector30″′ and made of a material having a moderate halogen etch rate, such asIn or Sb. With this arrangement, a plasma etch system can be deployed toetch debris from the plate 700 and location 702 on the collector 30″′,at approximately the same etch rate. As shown, the plasma etch systemcan include a source 144″ of plasma etchant and a controllable, RF powersupply 304′ that is attached through a capacitor to RF electrode 306′,as shown.

The system may further include an instrument 704 for analyzing etchingplasma emission from the witness plate 700. For example, the instrument704 may be a spectrometer. As shown, an optical fiber 706, e.g., fiberoptic cable can be used to transmit etching plasma emission from thewitness plate 700 to the instrument 704. Other suitable techniques forefficiently transmitted the etching plasma emission from the witnessplate 700 to the instrument may include a focusing optic, e.g., lens(not shown). For the etch control system, the instrument may produce anoutput indicative of a debris accumulation amount on the witness plate700. This output may then be received by a controller 708 which thenused the output to vary an etch rate parameter to control plasma etchrate. For example, the controller 708 can vary the RF power or theetchant concentration in the chamber 26.

To measure the amount of debris accumulation on the witness plate 700,the instrument may measure a spectral line intensity for the witnessplate material, e.g., In or Sb. If the witness material line intensityexceeds the highest allowable preselected value, the indication is thatthe etching efficiency exceeds the debris flux, e.g., Sn flux. In thiscase, the RF power or etchant concentration may be reduced by thecontroller 708. Alternatively, if the witness material line intensitybecomes smaller than the specified minimum value, the indication is thatthe cleaning power of the etcher is insufficient for the arriving debrisflux, e.g., Sn flux, and the RF power or etchant concentration may beincreased.

The witness plate material spectral line intensity may be used asfeedback to control RF power and/or etchant concentration to keep thewitness plate material spectral line intensity (as measured by theinstrument 704) at a specified level or within a specified range.Alternatively, a ratio of spectral intensities for the EUV plasmatarget, e.g., Tin, line and the witness material line can be kept at thespecified target value or within a specified range.

It will be understood by those skilled in the art that the aspects ofembodiments of the present invention disclosed above are intended to bepreferred embodiments only and not to limit the disclosure of thepresent invention(s) in any way and particularly not to a specificpreferred embodiment alone. Many changes and modification can be made tothe disclosed aspects of embodiments of the disclosed invention(s) thatwill be understood and appreciated by those skilled in the art. Theappended claims are intended in scope and meaning to cover not only thedisclosed aspects of embodiments of the present invention(s) but alsosuch equivalents and other modifications and changes that would beapparent to those skilled in the art.

1. An EUV metrology monitor for an EUV light source, said sourcegenerating debris by plasma formation, said monitor comprising: aradiation detector; an element for filtering radiation generated by theEUV light source and sending filtered radiation to said detector, saidelement positioned at a location wherein debris generated by plasmaformation is deposited on the element; and a heater to heat the elementto a temperature sufficient to remove at least a portion the depositeddebris.
 2. An EUV metrology monitor as recited in claim 1 wherein saidelement is a multi-layer mirror.
 3. An EUV metrology monitor as recitedin claim 2 wherein said multi-layer mirror comprises at least one layerof MoSi₂ and one layer of Si.
 4. An EUV metrology monitor as recited inclaim 1 wherein said element is a metal foil.
 5. An EUV metrologymonitor as recited in claim 4 wherein said foil comprises zirconium. 6.An EUV metrology monitor as recited in claim 1 wherein said heater isselected from the group of heaters consisting of an ohmic heater, aradiative heater, a radio-frequency heater and a microwave heater.
 7. AnEUV metrology monitor as recited in claim 1 wherein the plasma comprisesa plasma formation material, an etchant for the plasma formationmaterial is introduced into the monitor, and the heater heats theelement to a temperature greater than 200° C. to initiate a chemicalreaction between deposited plasma formation material and the etchant. 8.An EUV metrology monitor as recited in claim 7 wherein the plasmaformation material comprises Sn.
 9. An EUV metrology monitor as recitedin claim 7 wherein the etchant is selected from the group of etchantsconsisting of HBr, Br₂, Cl₂, HCl, H₂ and combinations thereof.
 10. Adevice for removing debris from an EUV light source collector mirror,said debris generated by plasma formation, said collector mirrorpositioned relative to a plasma formation site to cause a differentdebris deposition rate at different zones on the collector mirror, saiddevice comprising: a first heating system for heating a first zone ofsaid collector mirror to a first temperature, T₁, to remove debris fromthe first zone; and a second heating system for heating a second zone ofsaid collector mirror to a second temperature, T₂, to remove debris fromthe second zone, with T₁≠T₂.
 11. A device as recited in claim 10 whereinsaid first heating system comprises a heater selected from the group ofheaters consisting of an ohmic heater, a radiative heater, aradio-frequency heater and a microwave heater.
 12. A device as recitedin claim 10 wherein the collector mirror is located in a chamber, theplasma comprises Sn, an etchant is introduced into the chamber, and saidfirst temperature, T₁ and said second temperature, T₂ are each in arange of 150 to 400° C. to initiate a chemical reaction betweendeposited Sn and the etchant.
 13. A device as recited in claim 12wherein the etchant is selected from the group of etchants consisting ofHBr, Br₂, Cl₂, HCl, H₂, and combinations thereof.
 14. A system forprotecting a surface of an EUV light source optical element from debrisgenerated by plasma formation, the system comprising: a shieldcomprising at least one hollow tube having a tube wall that surrounds atube lumen, the tube being interposed between a plasma formation siteand the surface and oriented to prevent at least a portion of the debrisdirected toward the surface from reaching the surface and allowing atleast a portion of light generated at the plasma formation site to passthrough the lumen and reach the surface; and a heater for heating thetube wall to remove debris deposited thereon.
 15. A system as recited inclaim 14 wherein said heater is selected from the group of heatersconsisting of an ohmic heater, a radiative heater, a radio-frequencyheater and a microwave heater.
 16. A system as recited in claim 14wherein the plasma comprises a plasma formation material, an etchant forthe plasma formation material is introduced into the monitor, and theheater heats the element to a temperature greater than 200° C. toinitiate a chemical reaction between deposited plasma formation materialand the etchant.
 17. A system as recited in claim 16 wherein the plasmaformation material comprises Sn.
 18. A system as recited in claim 16wherein the etchant is selected from the group of etchants consisting ofHBr, Br₂, Cl₂, HCl, H₂ and combinations thereof.
 19. A system as recitedin claim 16 wherein the etchant is directed through the tube and awayfrom the detector surface.
 20. A system as recited in claim 14 whereinthe shield comprises a plurality of hollow tubes, with each tube havinga respective lumen and wherein each tube is oriented to allow lightgenerated at a plasma formation site to pass through the respective tubelumen and reach the detector surface, and wherein each hollow tube has arespective first end and a respective second end, a respective lumendiameter, d; and defines a respective linear tube axis from therespective first end to the respective second end, and wherein each tubeis formed with a respective midsection between the respective first endand respective second end, the respective midsection being laterallyoffset from the respective tube axis by an offset distance, D, with D≧d.21. A system as recited in claim 14 wherein the optical element isselected from the group of optical elements consisting of a detector andan imaging window.